Recently, as one example of usage of LEDs, the use of LEDs for an LCD (Liquid Crystal Display) backlight is becoming popular. Generally, when LEDs are used for an LCD backlight, light is emitted by applying a predetermined constant current to a plurality of LEDs connected in series. In this case, the number of LEDs and the amount of current are determined according to the amount of light required. Further, the drive voltage for driving LEDs is generated in a voltage converting circuit that converts a power supply voltage into a predetermined voltage. This voltage converting circuit controls the drive voltage by detecting the voltage values or current values of predetermined terminals of LEDs, which are loads, and feeding back these values. For example, Patent Literature 1 discloses the above-described LED drive technique.
The light emitting element drive apparatus disclosed in Patent Literature 1 will be briefly explained below using FIG. 1. This conventional light emitting element drive apparatus has a plurality of rows of LED array loads formed by connecting one or more LEDs in series. LED array loads 110 to 113 are driven by constant currents I0 to I3 generated by constant current sources 120 to 123 each connected to cathode terminals of LED array loads 110 to 113 in series. Cathode terminal voltages V0 to V3 of LED array loads 110 to 113 are received as input in selecting circuit 130. Selecting circuit 130 selects the minimum voltage of cathode terminal voltages V0 to V3, outputs this voltage as detected voltage Vdet and inputs detected voltage Vdet in controlling circuit 131. Controlling circuit 131 compares detected voltage Vdet and internal reference voltage Vref (not shown). Then, controlling circuit 131 inputs switching signal Cont for operating voltage converting circuit 100 to make detected voltage Vdet equal to reference voltage Vref, to the gate of transistor 103. Note that voltage converting circuit 100 is a boost converter formed with coil 101, diode 102, transistor 103 and capacitor 104, and converts predetermined input voltage Vdd into output voltage Vh required to drive LED array loads 110 and 113. Output voltage Vh is supplied to the anode terminals of LED array loads 110 to 113.
In the conventional light emitting element drive apparatus shown in FIG. 1, by individually driving constant current sources 120 to 123 on and off and adjusting the proportion of the duration of “on” and the duration of “off,” it is possible to adjust the amount of current that flows to LED array loads 110 to 113. By this means, it is possible to individually adjust the brightness of each LED array load. Further, selecting circuit 130 operates to control output voltage Vh of voltage converting circuit 100 according to the LED array load producing the maximum forward voltage drop among LED array loads 110 to 113 (that is, the LED array load having the minimum cathode terminal voltage among cathode terminal voltages V0 to V3). Consequently, it is possible to drive LED array loads 110 to 113 with little loss, while securing sufficient light emission from LED array loads 110 to 113.